Game concept
De Jure Capital
From the game's encyclopedia
The De Jure Capital of a Title is the County that was its historical capital. You can always choose to move your Realm Capital to your Primary Title's De Jure Capital, even if it has already been moved.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
De Jure Capital is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "The De Jure Capital of a Title is the County that was its historical capital." Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.